Opposition MPs in South Australia are demanding to know how much Lance Armstrong is being paid to ride in the Tour Down Under after the cyclist endorsed Premier Mike Rann's election campaign.

Anger has been mounting since Armstrong, in Adelaide for the Tour, declared that 'Rann is my man.' Mr Rann's government is refusing to say how much Armstrong has been paid to cycle there, claiming secrecy is necessary to stop other states "stealing" him away.

Independent MP David Winderlich has lashed out, saying Armstrong's fee was "backdoor" political advertising, while Opposition sports spokesman Terry Stephens said the people should decide whether the seven-time Tour de France winner was worth the money.

According to The Australian newspaper, Mr Stephens cited the Victorian government's upfront disclosure that golfer Tiger Woods was paid $1.5 million to play in the Australian Masters last November.

"Premier Rann is keen to be seen with these sporting celebrities, but is not so keen on being frank with SA taxpayers," Mr Stephens said.

Rann government minister Jay Weatherill yesterday revealed cabinet had not been informed about payments to Armstrong or tennis legend John McEnroe, who also made a public appearance with Mr Rann this week.

But Tourism Minister Jane Lomax-Smith said disclosing Armstrong's fee - which has been speculated at between $1m-$2m - would put the Tour Down Under at risk. "There are many people who'd like to steal our major events and our sports and arts activities," she said.

"We don't want to help those people who we know are sniffing around by giving them an idea of how they're managed."

The guy is just another cyclist. It's sad that his opinion of an Aussie politician should be worth so much. I don't mind them paying him to race but we'd all be happier if he stuck to talking about the race, his team and cancer.

If he wants to go into politics he should quit his job and lay his cards on the table. Based on events during last year's TdF I'd say his diplomacy skills need a some serious polishing for a start.